The present invention relates to integrated circuit manufacture. More particularly, the present invention relates to the post-etch passivation of metalized layers within semiconductors.
Solid state devices, including semiconductors and integrated circuit devices (ICs) are manufactured in four distinct stages. They are material preparation, crystal growth and wafer preparation, wafer fabrication, and packaging. Wafer fabrication is the series of processes used to create the semiconductor devices in and on the wafer surface. The polished starting wafers enter fabrication with blank surfaces and exit with the surface covered with hundreds of completed chips.
Wafer fabrication facilities produce billions of chips world-wide, with thousands of different functions and designs. Even with this daunting diversity of device types, the basic processes which are used to form solid state devices are no more than four: layering, patterning, doping, and heat treatment.
Each of these broad processes may be further broken down. One means of patterning one or more layers formed on of a wafer during fabrication is etching. Patterning is often commenced by laying down a mask layer. One type of mask is a photoresist. Etching can be used, in conjunction with other steps such as photoresist deposition, to form a variety of features through one or more layers in an integrated circuit or other solid state device. Some of these layers are metalized, and aluminum is one metal used to form these metalized layers.
The etching of aluminum presents problems that must be overcome in order to reliably produce commercial quantities of solid state devices which are reliable and which achieve the design objectives for the device. During aluminum etching, chlorine is incorporated into the photoresist and a relatively xe2x80x9cchlorine-richxe2x80x9d material is deposited on the aluminum sidewalls; this situation is represented graphically in FIG. 1. The specific composition, thickness, and amount of Cl incorporated in the sidewall deposition is highly dependant on factors such as the type of photoresist and the etch chemistry.
In some cases the sidewall deposition can be relatively thin, e.g. Cl2/BCl3 etching with I-line resist. In other cases the sidewall deposition can be hundreds of angstroms thick, for instance Cl2/BCl3/CHF3 etching with DUV resist. For thicker depositions, the typical corrosion passivation processes can result in a situation in which the entire thickness is not depleted of chlorine. Instead, a Cl-depleted region may be formed on an outer layer as a result of the sidewall passivation. This region may be substantially chlorine-free but a xe2x80x9cchlorine-richxe2x80x9d rich region exists between the depleted surface and the aluminum line. See FIG. 2.
In order to limit the deleterious effects of corrosion, especially ongoing corrosion, within the solid state device, one of the steps commonly taken is corrosion passivation. This is especially true of metals which tend to form substantially impermeable oxides, such as aluminum.
In general, corrosion passivation results from allowing the corrosion process to begin while breaking the cycle before a significant amount of corrosion can form. Most prior corrosion passivation procedures depend on a H2O-based plasma to react with residual chlorine to form HCl, which is removed from the wafer surface. The photoresist is stripped off with an O2-based plasma process either concurrently or in a subsequent processing step. Ideally, this sequence will remove essentially all of the residual chlorine on the wafer. However, it has been observed that such current passivation methodologies may not remove all the chlorine on the surface of a wafer. Accordingly, they may result in a low xe2x80x9ccorrosion marginxe2x80x9d, or the window of time before formation of detectable amounts of corrosion. This is especially true for xe2x80x9cnext generationxe2x80x9d aluminum etch processes which involve DUV photoresists and aluminum etch processes containing CHF3 or N2, all of which result in relatively thick deposition on the aluminum sidewall.
In order to investigate a more effective passivation methodology, it is well to study the formation of xe2x80x9cclassicxe2x80x9d corrosion. An overview of one corrosion mechanism of significant concern in the solid state industry, including a general mechanistic sequence follows. The exact mechanisms for the formation of classic corrosion are complicated and have not been completely elucidated, but a reasonably general mechanistic sequence, with an identification of the critical factors, is presented below.
It is known that substantially any chlorine remaining on the aluminum wafer after the passivation process will result in the formation of corrosion when the wafer is exposed to ambient humidity.
1) Transport of Water to the Wafer Surface: The first step in the formation of corrosion occurs when water from the ambient environment diffuses to the surface of the wafer. The flux of water to the wafer, and the resulting equilibrium surface H2O concentration, will be controlled by the absolute concentration of water in the vapor: i.e., the higher the ambient water concentration, the greater the surface concentration of water.
2) H2O Diffusion. Before corrosion can form, the water on the surface of the wafer must diffuse to the Cl-rich region. The rate of water diffusion will be effected by temperature. The higher the temperature, the faster the diffusion. The amount of water diffusing into the sidewall will be controlled by the equilibrium concentration of water on the wafer surface. The higher the surface concentration, the larger the H2O flux to the corrosion site.
3) The Corrosion Cycle Begins. Water reacts with the residual Cl to form HCl, which further reacts to form corrosion. A typical reaction scheme is presented in Equations1-3:
AlCL3+3 H2Oxe2x86x92Al(OH)3+3HClxe2x80x83xe2x80x83(I)
Al(OH)3+3HCl+3H2Oxe2x86x92AlCl3xc2x76H2Oxe2x80x83xe2x80x83(II)
2AlCl3xc2x76H2Oxe2x86x92Al2O3+9H2O+6HClxe2x80x83xe2x80x83(III)
The rate at which HCl is formed will depend on the concentration of residual chlorine and the amount of water that has diffused through the film. Note that water plays a key role because it acts as a catalyst for the overall corrosion reaction. The rate of each reaction is strongly dependant on the temperature the higher temperature, the faster the reaction. Moreover, while a typical reaction scheme is shown here, other reaction sequences that form corrosion may also be present. The invention taught hereinafter is not necessarily dependent on any one of these reaction sequences.
4) The Corrosion Cycle Accelerates. HCl reacts with pure aluminum to form ALxCly, which subsequently reacts with H2O to form corrosion and more HCl, see Equation IV below.
3HCl+3H2O+Alxe2x86x92AlCl3+3H2Oxe2x86x92Al(OH)3+3HClxe2x80x83xe2x80x83(IV)
This cycle continues until the corrosion site breaks through the sidewall passivation and continues to grow on the outside of the aluminum line, as shown at FIG. 3. As the local concentration of water and HCl increases, the amount and rate of corrosion formation also increases. The corrosion cycle will continue for as long as there are present H2O, Cl, and Al, which form the reactants. See FIG. 4.
The typical passivation procedure occurs at conditions that serve to impede the diffusion of water and hence reduce the effectiveness of the passivation process. Specifically, these inefficiencies are as follows:
1) The entire passivation sequence is carried out at low pressure. The typical pressure range s 2-4 Torr. At these low pressures the concentration of water in the chamber is relatively low, which reduces the amount of water transported to the wafer surface, and ultimately lowers the flux of water to the Cl-rich region. This has the effect of slowing the passivation process.
2) The entire passivation sequence is carried out at high wafer temperatures. The wafer temperature range for the typical passivation methodology is 220-275xc2x0 C. These high temperatures have the effect of driving off H2O as well as HCl, which results in a reduction of the amount of water available for participation in the passivation process. This also has the effect of slowing the passivation process.
3) The passivation process is typically a plasma process. The plasma for the typical passivation methodology either consists solely of H2O or a mixture with typical photoresist strip gasses including but not limited to O2, N2, and CF4. The products of the plasma process have the effect of oxidizing the metal incorporated into the sidewall passivation during the aluminum etch, which can result in the creation of an xe2x80x9coxidized skinxe2x80x9d on the outer surface of the sidewall passivation. See FIG. 2. This can have the effect of trapping chlorine beneath the skin, thereby setting the stage for future corrosion and decreasing the effectiveness of the H2O passivation. This is because the H2O must diffuse through the oxidized layer before reacting, and the HCl must diffuse out before it can be removed.
Based on this conceptual understanding, what becomes clear is that temperature and absolute concentration of water in the vapor are critical parameters for the formation of corrosion, and hence for corrosion passivation. Accordingly, it is for the formation of corrosion, and hence for corrosion passivation. Accordingly, it is desirable to control at least one of temperature and absolute concentration to perform a more complete corrosion passivation.
The present invention teaches a two-step process which maximizes the efficiency of chlorine conversion and removal, and hence corrosion resistance. The two steps thereof are:
Surface Saturation, which preferably occurs at conditions of relatively high pressure and low wafer temperature, with no plasma. The high pressure will maximize the concentration of water in the chamber, while the low wafer temperature will allow the surface of the wafer to become saturated. Surface wafer saturation tends to maximize the rate and amount of water diffusing into the sidewall passivation. The lack of plasma exposure prevents the formation of diffusion-inhibiting xe2x80x9ccrustxe2x80x9d layers. The timing of this step can be varied depending on the amount of residual chlorine present on the wafer, i.e. sidewall passivation thickness.
After surface saturation, a Corrosion Cycle xe2x80x9cQuenchxe2x80x9d is performed where the pressure in the reaction vessel is quickly ramped down, and the wafer temperature is quickly ramped up, again with no plasma. The combination of the pressure drop and the concurrent temperature rise result in the rapid removal of both the water and the HCl from the wafer surface and hence breaks the corrosion cycle. The rate and setpoint which the temperature ramps up to, and the rate and setpoint the pressure ramp down to, represent variables which are used to control how quickly the corrosion cycle is halted. The temperature ramp may be achieved with the use of heat lamps, or some other Rapid Thermal Process methodology, and the pressure ramp controlled, for instance, by controlling the chamber throttle valve or pumping speed.
These and other advantages of the present invention will become apparent upon reading the following detailed descriptions and studying the various figures of the Drawing.